This invention relates to woven papermakers' fabrics and especially to forming fabrics, including those known as fourdrinier belts or fourdrinier wires.
In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulosic fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless belt of woven wire and/or synthetic material. The belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium by providing for the drainage of the aqueous medium through mesh openings, also known as drainage holes, by vacuum means or the like located on the machine side of the fabric to form a wet paper web. After leaving the forming section, the wet paper web is transferred to the press section of the machine, where it is passed through a series of pressure nips formed by cooperating press rolls to remove still more of its moisture content and finally to the dryer section for further moisture removal.
Such papermakers' fabrics are manufactured in two basic ways to form an endless belt. First, they can be flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form the endless belt. Alternatively, they can be woven directly in the form of a continuous belt by means of an endless weaving process. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In a papermakers' fabric having been woven in an endless fashion, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein the terms "machine direction" and "cross machine direction" refer respectively to a direction equivalent to the direction of travel of the papermakers' fabric on the papermaking machine and a direction transverse to this direction of travel. Both methods are well known in the art and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet support and lack of wire marking are important considerations in papermaking, especially in the formation of the wet paper web. The problems of sheet support and wire markings are particularly acute in the formation of fine paper grades where the smoothness of the sheet side surface of the forming fabric is critical as it affects paper properties such as sheet mark, porosity, see through, pin holing and the like. Accordingly, paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing and like grades of fine paper have heretofore been formed on very fine woven forming fabrics or fine wire mesh forming fabrics. Such forming fabrics, however, are delicate, lack stability in the machine and cross machine directions, and are characterized by relatively short service lives due to abrasion and wear caused by contact with the papermaking machine equipment.
In short, in order to ensure good paper quality, the side of the papermakers' fabric which contacts the paper stock should provide high support for the stock, preferably in the cross machine direction because support is already provided in the machine direction, to reduce wire marking and enhance smoothness. Conversely, the side of the papermakers' fabric which contacts the rollers and machine must be tough and durable. These qualities, however, most often are not compatible with the good drainage and fabric characteristics desired for a papermakers' fabric.
In order to meet both competing standards, fabrics have been created using multiple warps, so that the fabric would have the desirable papermaking qualities on the surface that faces the paper web and desirable abrasion resistance properties on the machine contacting surface. For example, papermakers' fabrics may be produced from two different fabrics, one having the qualities desired in the paper contacting side and the other with the qualities desired in the roller contacting side and then the two fabrics are joined together by a third set of threads. This type of papermakers' fabric is commonly called a triple layer fabric. Alternatively, two layers of fabric can be woven at once by utilizing threads of different sizes or of different materials with one set of threads which is part of one of the weaves to bind the layers together. This fabric is commonly called a double layer fabric. The problem with both these papermakers' fabrics, however, has been that the thread which interconnects the two layers forms undesirable knuckles. Often these knuckles are pronounced on the fabric surface due to the angles their paths form. With use, the knuckles degrade the quality of the paper formed and snag as the fabric slackens.
It is therefore an object of the present invention to provide a papermakers' fabric with a superior papermaking surface, good drainage characteristics and resistance to abrasion and wear.
Another object of the present invention is to provide a papermakers' fabric with yarns having a reduced path deflection to produce a smooth sheet forming surface.
Yet another object of the present invention is to provide a papermakers' fabric with an increased structural rigidity and wear resistance.